Anytime good-ole southern boys get to talkin’, the conversation usually migrates to cars — driving, bragging, and racing. I know this because I’m a good-ole southern boy.
We tell the same stories over and over though — the only thing that changes is the extent of the embellishment. My friend Mark always brags about his car’s explosive acceleration. Every intersection and green light is his imaginary drag strip. My other friend Mike is always buying some new device that increases his car’s horsepower. With all the gadgets he’s bought over the years, his car must have 3,000 horsepower by now. And Brad, who’s still a youngster, tells tall tales of eluding the police and impressing the girls with his wheels.
We become “one” with our cars when weekend TV switches from football to NASCAR and Indy racing, so much so you’d think driving is something we do professionally. We beat the other car off the line. We brake to a screeching halt. We swerve around traffic. We snooker the cops and vanish from the radar gun. But it isn’t real.
You see, we talk like we control the road, but in reality, the road, the traffic, and the hazards control the way we drive.
In industry — a refinery, a steel mill, or municipal water plant hires someone and designates them a “Pump Operator.” This person is under instructions to operate the pump. In reality though, he doesn’t really “operate” the pump.
His real function is to negotiate system upsets and protect the pump from damage. This is because the system is what actually operates the pump. The pump takes what the suction gives it and jacks-up the pressure, with power through the impeller, tolerances and velocity, to meet the resistance in the discharge piping. The pump is the passive receiver of the system’s demands.
If the process and production engineers understood it this way, and taught it this way to the pump operators, we’d have less “Bad Actor pumps,” less mysterious vibrations, and longer seal and bearing life on rotating equipment. But these are issues for another article (I digress).
By saying the system controls the pump, I mean the pump will do what the system makes it do. If the system makes the pump do what it cannot, then the pump will shut down or fail by taking out its bearings and/or seals.
In order to have the best pump installed into the system, you must first understand the requirements of the system. There are up to four different requirements in each system. These four requirements are simply added together to determine the pump best suited for the system. These requirements are called the TDH, or Total Dynamic Head. The formula is: TDH = Hs + Hp + Hf + Hv, where:
• Hs is the static head or elevation change across the system.
• Hp is the pressure head or pressure differential across the system.
• Hf is the friction head or resistance in the pipes, valves,and fittings.
• Hv is the velocity head or losses due to the liquid’s velocity through the pipes.
Of the four elements of the TDH, the Hs and Hp are determined by observation. The Hf and the Hv can either be “guesstimated” or measured. You’ll have to come to my seminar to understand the term “guesstimate”. It’s somewhat complicated for this article. (See the promo box at the end of this article for more details on the Pump Guy Seminar Series.)
In fact, the Hf and the Hv are the reason that design engineers are contracted to specify pumps before the construction phase of a new plant. And the Hf and Hv are also the reasons for malpractice lawsuits against the design engineers later.
A young design engineer wrote to me last week. He was about to make this precise mistake:
Dear Pump Guy,
I have taken on a project to provide fresh water to draught affected areas in arid climates by desalinating seawater. I’ve been looking at pumps to transport the liquid, and I have decided I need 100 litres/minute with a head of 100 meters. I have come across very little resources on pumps on the Internet, and I am not very familiar with the concept of “pressure head.” If I had to elevate the seawater 100 m, could I use two pumps that have heads of 50 meters? Any help would be appreciated, as well as links to any helpful resources you may know of.
Thanks for your help,
You see, this young engineer is about to specify a pump based solely on the elevation change across the system. He is hazy on the concept of pressure head (Hp) and hasn’t considered friction and velocity losses (Hf & Hv) across the system. This is the beginning of a classic malpractice lawsuit.
You must consider all four elements in every system and understand why and how they are dynamic. That’s why it is called the TDH. And, mate the pump into the dynamic system.
You’ll want the pump’s best efficiency point (BEP) to meet or equal the TDH. If the BEP = TDH, the pump will run for many years with minimal maintenance. Problem is, once a plant is commissioned and production begins, the TDH goes ballistic.
In the short term, levels change in the tanks, pressures go up and down, valves open and close, and filter screens clog. As maintenance occurs, pipe schedules are changed, equipment is changed, and new equipment is added into the system.
In the long term, equipment loses efficiency, scale forms on the internal pipe walls, and the plant undergoes expansion and contraction with the economy. And through all this, the pump has a static BEP. BEP needs to be replaced by the BER …”Best Efficiency Range.” We spend a lot of time on this at my seminars. In class, we develop the pump’s BER. Then we develop the dynamic system curve and show how to mate the pump into a dynamic system.
You must dominate these concepts or they will dominate you, your pumps, and your maintenance budget. Learn to use them or they will mess with your pumps and reliability. Put them in your CHEAT SHEETS.
For more "Cheat Sheets" articles, go here.
Larry Bachus founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control magazine. He is a pump consultant, lecturer, and inventor based in Nashville, Tenn. Mr. Bachus is a member of ASME and lectures in both English and Spanish. He can be reached at [email protected] or 615 361-7295.